Locomotives (or other vehicles) typically include a prime mover that is powered by a fuel source to generate mechanical energy. In one example of a locomotive, mechanical energy generated by the prime mover may be converted to electrical energy that is used to power traction motors and other components and systems of the locomotive. In some examples, the prime mover may be a combustion engine that is fueled by diesel, gasoline, or other liquid petroleum distillates. In other examples the engine may additionally or alternatively utilize a gaseous fuel, such as natural gas.
In light of its favorable energy content, liquefied natural gas (LNG) may be used as a fuel source for a locomotive prime mover. Particularly for long-haul applications, it may also be desirable to utilize a fuel tender for carrying one or more LNG storage tanks. The fuel tender may be coupled to the locomotive prime mover. Prior to injection into a locomotive's natural gas-fueled engine, LNG is typically vaporized into gaseous natural gas (CNG).
In one example, when LNG is stored in an unrefrigerated tank, the LNG storage tank gradually heats up due to tank insulation not being 100% efficient. Even with efficient insulation and cryogenic refrigeration equipment, heat may leak into the LNG storage tank. Additionally, over time, the LNG tank pressure builds to a safety relief pressure threshold, thereby causing a pressure relief valve to open. Opening of the pressure relief valve vents the LNG storage tank and reduces the LNG tank pressure. However, venting the LNG storage tank may waste fuel while also presenting a safety risk, including adding pollutants to the environment.
In some examples, the LNG storage tank system may include an economizer for controlling a pressure within the LNG storage tank. Specifically, when the LNG storage tank pressure increases above a threshold pressure, vapor within the LNG tank may be preferentially routed to a vaporizer in order to utilize the higher pressure gas. However, this system does not allow the LNG tank pressure to be reduced below the threshold pressure for periods of inactive use.